In a conventional third generation partnership project (3GPP) system, there are four non-idle radio resource control (RRC) states roughly corresponding to four levels of WTRU activity: a dedicated channel (DCH) cell level (Cell_DCH) state, a forward access channel (FACH) cell level (Cell_FACH) state, a paging channel (PCH) cell level (Cell_PCH) state, and a universal terrestrial radio access network (UTRAN) registration area (URA) PCH (URA_PCH) state. In a Cell_DCH state, a WTRU has a dedicated physical channel for data transport. In a Cell_FACH state, no dedicated channel is allocated to the WTRU, but the WTRU may use a random access channel (RACH) and a FACH channel for conveying and receiving signaling as well as user plane data. It is not efficient to send a large amount of user plane data in the Cell_FACH state. A Cell_PCH state reduces battery consumption by only listening to a PCH in a discontinuous reception (DRX) mode. As with the Cell_DCH and Cell_FACH states, the location of a WTRU in the Cell_PCH state is known at the cell level. A WTRU in the Cell_PCH state temporarily enters a Cell_FACH state when it relocates to a new cell in order to communicate its new location information. A URA_PCH state is similar to the Cell_PCH state, except that in the URA_PCH state the network is only informed when the WTRU moves to a new URA. When a WTRU changes cells, the WTRU generally stays in the same state. Currently, handovers in the Cell_DCH state are network-directed.
A WTRU that is in an active state has a non-access stratum (NAS) connectivity so that the WTRU may communicate to nodes in a core network. A WTRU in an active state also has an access stratum (AS) connectivity such that a radio bearer configuration, (e.g., WTRU capability exchange, ciphering, or the like), has been completed for the WTRU.
A WTRU in an idle state consumes less power and resources than a WTRU in a low-power active state. One important characteristic of a WTRU in an idle state is that the WTRU does not have to participate in an active mode handover. In other words, when a WTRU in an idle state moves from one cell to another, the WTRU does not configure radio bearers with the new cell if the WTRU remains in an idle state.
One of the goals in a next generation wireless communication system is maintaining an “always on” connectivity. However, for a battery-powered WTRU, battery power consumption is an issue. The “always on” connectivity is a desirable feature, but this tends to shorten the battery life.
Currently in 3GPP, a WTRU maintains uplink synchronization whenever it has a dedicated channel to a base station. The WTRU always maintains uplink synchronization in a Cell_DCH state. The WTRU also resynchronizes its uplink any time it has a new set of dedicated channels disjoint from its prior set. Maintaining uplink synchronization, (conventionally via RACH transmissions), is one of the sources for consuming the battery power of the WTRU.
Therefore, it would be desirable to provide a scheme for maintaining uplink synchronization efficiently and reducing battery power consumption while the WTRU is in an active state.